Rubber-repellent coated paper



Patented Sept. 26, 1933 UNITED STATES PATENT OFFICE RUBBER-REPELLENT COATED PAPER No Drawing.

Application May 16, 1930 Serial N0. 453,097

6 Claims.

This invention relates to a coating composition, to paper coated therewith having rubber-repellent properties adapted to be used as a backing for sheets, strips or films of rubber, and for the purpose of preventing the same from sticking together, and to a process of preparing such coated paper. The invention also includes articles formed by attaching sheets, strips or films of rubber to the said coated paper as a backing, as

- strips or films of rubber and an interposed layer or layers of the coated paper.

It is common practice in the rubber industry to ship or store rubber goods, especially uncured or semi-cured rubber goods, in the form of a pile of superposed sheets. In order to prevent the sheets of rubber from sticking together it is customary to place a piece of rubber-repellent paper between them. This paper must be strong, must notfcrack on bending, .and must be capable of being cleanly peeled oil of the rubber after being subjected to considerable pressure. Kraft paper coated with a solution of ordinary water-glass has been used for this purpose. Although such coated paper is rubber-repellent to a certain degree, it is open to the objection that it is very brittle and has low tearing and folding strength. The object of this invention is to provide a repellent properties and which fulfills the abovenoted requirements more successfully than rubber-repellent paper heretofore used.

In making the coated paper of this invention I make use of a partially neutralized water-glass solution. I have found that the controlled addition of acid to water-glass greatly improves its properties as a coating for the production of rubber-repellent paper. Presumably the high alkalinity of the commercial grades of waterglass is largely responsible for the brittleness and low tear and fold strength of paper coated therewith. By reducing this alkalinity by the careful addition of an acid I can retain the rubber-repellent properties, and atthe same time greatly improve the paper with respect to strength and brittleness. The amount of acid added must be rather carefully governed, for although there is a progressive decrease in the brittleness of the paper with increasing amounts of acid used, there 5 comes a point where the water-glass sets to a gel which cannot be successfully applied to the paper.

For example, I have found that a mixture of cc. of ordinary 40 B. water-glass diluted with 50 cc. of water, to which are added 50cc. of norwell as articles comprising two or more sheets,.

mal sulfuric acid, sets to a gel in sixteen hours; when cc. of normal sulfuric acid are added the mixture sets to a gel in fifty-five minutes; when '70 cc. of this acid are added the mixture sets in eight minutes; and when 80 cc. of this acid are 60 added the mixture sets immediately. I have obtained excellent results with the solution mentioned above which sets in sixteen hours at room temperature.

The following table shows a comparison of the coated paper which possesses excellent rubber- Test Fold Sample Bursting strength Cross Machine Cross Machine direction I direction direction direction A .36 80 I 104 1016 601 57 64 l 80 250 274 71 82 101 1090 800 The coating composition may be applied by any suitable method, for instance, by any of the known methods used for coating paper. The coating operation may also be carried out with tub sizing equipment. The paper may be coated on one or both sides; for some purposes it is suflicient to coat one side, whereas for others it is desirable to have both sides coated. Where the paper is contacted on both sides with a layer' of rubber, it shouldbe coated on both sides.

As an example of my improved process for making rubber-repellent paper, I coat both' sides of a kraft paper with a mixture made by diluting ten gallons of 40 B. water-glass containing about 7.8% NazO with ten gallons of water and then adding ten gallons of 5% sulfuric acid with vigorous stirring. A coating of twelve pounds, dry weight, on a fifty-pound kraft paper gave a very satisfactory product. (The weights here refer to the weight of 500 sheets 25"X38) The product thus obtained is rubber-repellent, strong, and is not appreciably more brittle than untreated kraft paper. The paper may be calendered after being coated to improve its appearance, or it may be used in the uncalendered condition.

It is to be understood that the invention is not limited to the proportions of ingredients cited in the foregoing example, but that it embraces the use of water-glass which has been neutralized to such an extent that the mentioned properties of paper coated therewith are appreciably improved as compared with the properties of paper coated with the water-glass in its unneutralized state. Due to the complex composition of water-glass it is not possible to define the coating agent used in this invention by its percentage composition nor by the extent of neutralization. The invention embraces the use of any commercial water-glass which has been substantially improved as a coating agent by the addition of an acid or equivalent agent. In general the more acid that can be added without causing too rapid jellation, the better the quality of the coated paper produced therewith. The upper limit of acid added is determined by the time required for the mixture to jell. The mixture should not jell too soon after it is made up because in mill operations it would not ordinarily be practical to use a mixture which jells in less than say one-half an hour.

The difficulty in defining the percentage composition of the coating agent is due to the fact that there are a large number of commercial grades of sodium silicate on the market which vary in N a2OSiO2 ratios'and in solid contents. The silicate having a molecular ratio of 1 NazO to about 4 SiOz and a solid content of about 37% is the grade which contains the highest ratio of $102 to NazO produced commercially and this is the grade referred to as commercial sodium silicate in the above table. This silicate is, therefore, the least alkaline of the commercial silicates. It would, of course, be possible to manufacture a sodium silicate solution of still lower Na2OSiO2 ratio, but it'would be difficult or impossible to attain a satisfactorily high solid content in such a solution, and its shipment would, therefore, be expensive due to its large water content.

We have found that-a solution which has been neutralized to such an extent that it has an NazOSiOz ratio of about 1-4, and a normality with respect to NazO of about 1.4, gives excellent results. Such a solution is made up by mixing, for example, 2000 cc. of water with 2000 cc. of commercial water-glass and adding 600 cc. of one-tenth normal sulfuric acid. It is preferred to use a sodium silicate having a normality with respect to NazO of less than about 1.4.

The water-glass solution may be neutralized to a different extent depending upon the character of the paper which is coated therewith. When one uses a heavier kraft paper a correspondingly less neutralized silicate may be used and still a useful product will be obtained. In this case, however, one is relying on the greater strength of the heavier paper to give the desired strength to the finished product. Since the paper is the expensive item in the product, it is 'light a paper as possible.

' The invention is not limited to the use of any a particular acid. Sulfuric, hydrochloric, hydrofluoric, carbonic acids may be cited as'examples of acids which are suitable for the-purposes of this invention. The term acid is used herein not only in the strict chemical sense, but it is to be understood as including any acid reacting agent or any agent capable of neutralizing an alkali; for example, it includes acid anhydrides, like CO2, and acid salts, like sodium bisulfate. No particular grade of paper is necessary. Kraft paper is a desirable grade when high strength is required, but any other suitable grade may be used.

Although it is preferred to use a partially neutralized sodium silicate solution, it is to be understood that a partially neutralized potassium silicate solution may-also be employed, and in the appended claims the term water-glass is used to include both sodium'and potassium silicate solutions.

If desired, suitable filling material such as for example clay, may be added to the partially neutralized water-glass and the resulting composition then applied to the paper. An addition of 10% dry weight of clay, based on the dry weight of the water-glass, gives a very satisfactory coating composition, but it is to be understood that this proportion is cited by way of example only, and that the amount of filling material added may vary within wide limits.

I claim:

1. Paper coated with partially neutralized water-glass, said coated paper having rubber repellent properties and having substantially the same degree of pliability as the uncoated paper.

2. Paper coated with a partially neutralized water-glass solution having a normality with respect to NazO of less than about 1.4, said coated paper having rubber repellent properties and being not substantially more brittle than the uncoated paper.

3. A pliable, rubber repellent paper comprising paper coated with a water-glass solution which has been neutralized to such an extent that it gels within sixteen hours but not immediately at room temperature.

4. Process of making rubber repellent paper which comprises partially neutralizing water glass with acid and then applying the partially neutralized water glass to paper.

5. Process of making rubber repellent paper which comprises partially neutralizing water glass with acid until the resulting liquor has a normality with respect to NazO of not more than about 1.4, and then applying the partially neu-' tralized water glass to paper.

6. Process of making rubber repellent paper which comprises partially neutralizing water glass with acid to such an extent that it gels within 16 hours but not immediately at room temperature, and then applying the partially neutralized water glass to paper.

HAROLD W. WALKER. I 

